The present disclosure relates generally to steam quality meters, and in particular, to steam quality meters with electrodes for sensing capacitance and/or impedance.
Steam is commonly used for heating purposes in the process industry and as an injection fluid for recovering hydrocarbons in the oil and gas industry. Steam flow applications such as these utilize saturated steam, which can be all vapor, all liquid, or a combination of vapor and liquid. Thermodynamic relationships use steam quality to calculate properties of saturated steam, such as density and mass flow rate and enthalpy flow rate. These thermodynamic relationships assume that the saturated steam is a homogenous mixture of liquid and vapor phases coexisting at the saturation pressure and temperature, but in practice, saturated steam is rarely completely mixed. Therefore, a device such as a Venturi is used to mix the fluid so that homogeneity can be assumed. The thermodynamic relationships also assume a known steam quality. Density and mass flow rate calculations are often performed assuming a steam quality of 1.0, or 100%. However, for steam quality from 0.9 to 1.0, for every 1% reduction in steam quality, the density of homogenous steam increases by roughly 1%. As steam quality further decreases, the increase in density for every 1% decrease in quality becomes larger. Thus, assuming an incorrect steam quality can result in significant errors when calculating mass flow rate.
At a given pressure, the temperature of a steam mixture remains constant at the saturation temperature until all of the vapor becomes liquid or all of the liquid becomes vapor. Therefore steam quality cannot be determined by only measuring temperature and pressure. As a result, steam quality meters use means such as light, capacitance, impedance, or other methods to determine steam quality. Capacitance of the steam mixture varies with the percentage of liquid water in the mixture. The quality of the mixture is determined under the assumption that water droplets remain uniformly suspended in the vapor as the steam flow passes through horizontal pipe sections. To achieve a uniform homogeneous distribution of water droplets, a mixing device (such as a Venturi, a nozzle, a perforated plate, a swirling device, or other such element) is used. However, water droplets may settle disproportionately at the bottom of a steam pipe or towards the downstream end of the pipe as a steam mixture travels through the pipe. Current steam quality meters do not account for settling of water droplets.